1. Field of the Invention
The present invention relates in general to a method of controlling the rotational speed and phrase of a motor, and more particularly, to a method of controlling by use of software, operations of a drum motor employed in a Video Cassette Recorder (VCR).
2. Description of the Prior Art
Generally, a servo system is employed in apparatus requiring a precise control. One embodiment of a conventional servo system for providing such precise control will hereinafter be described with reference to FIG. 1.
The block diagram of FIG. 1 shows the construction of a conventional servo system for controlling rotational speeds and phrases of a capstan motor and a drum motor employed in a VCR. As shown in the drawing, the conventional servo system comprises a motor M, a phase control circuit 100, and a rotational speed control circuit 200.
Phase control circuit 100 includes a first amplifier 31 for amplifying a phase signal outputted from motor M by a predetermined amount of amplification, and a phrase error detector 32 for detecting the phase difference between a predetermined reference phase signal and the phase of an output signal from the first amplifier, and generating a phase error voltage in accordance with the detected phase difference.
Rotational speed control circuit 200 is provided with a frequency generator 33 for detecting a rotational speed of the motor M and generating a frequency signal in accordance with the detected rotational speed, a second amplifier 34 for amplifying the output signal from frequency generator 33 by a predetermined amount of amplification, and a rotational speed error detector 35 for generating a rotational speed error voltage in response to the output signal from second amplifier 34.
Also, the conventional servo system comprises an adder 36 for adding an output signal from the phase error detector 32 to an output signal from the rotational speed error detector 35, a compensating circuit 37 for inputting an output signal from adder 36 and processing the inputted signal to enhance the performance of the servo system, and a third amplifier 38 for amplifying an output signal from the compensating circuit 37 by a predetermined amplification amount to generate a control signal and applying the control signal to the motor M. The motor servo-operates based on the control signal from third amplifier 38.
N pole and S pole magnetic elements 39 and 39a are mounted on both sides of motor M to detect the phase signal from the motor inputted to first amplifier 31. Thus, as the motor rotates, the N pole and S pole magnetic elements 39 and 39a rotate together with the motor, thereby producing a magnetic variation. A pulse generator (not shown) generates a pulse signal in accordance with such magnetic variation. This very low pulse signal is amplified to a recognizable level by first amplifier 31.
In a conventional VCR system, a vertical synchronous signal in a composite video signal, with the frequency divided by 1/2 by a frequency divider (not shown), has been typically used as the reference phase signal, but with different values possibly used in accordance with the particular capstan motor or drum motor. Also, different values may be used depending on whether the unit is operating in the recording mode or the playback mode.
Compensating circuit 37 usually employed in the control system serves to increase a DC gain to compensate for the steady state error and a load suppression characteristic, and/or is adapted to filter a signal inputted therein to remove external noise and distortion components. Therefore, compensating circuit 37 has various different constructions depending on the control system, and the circuit usually includes an operational amplifier with an external resistor and capacitor.
The operation of the conventional servo system with the above-mentioned construction will now be described.
First, upon receiving a phase signal from motor M and a reference phase signal, phase error detector 32 in phase control circuit 100 detects the phase difference between the reference phase signal and the phase signal from the motor and generates a phase error voltage in accordance with the detected phase difference.
On the other hand, frequency generator 33 in the rotational speed control circuit 200 generates a frequency signal in accordance with the number of rotations of motor M. This very low frequency signal is amplified by second amplifier 34 by a predetermined amplification degree or amount. Rotational speed error detector 35 generates a rotational speed error voltage in response to the output signal from second amplifier 34.
Then, adder 36 adds the phase error voltage from phase error detector 32 to the rotational speed error voltage from rotational speed error detector 35 and outputs the added value to compensating circuit 37 Compensating circuit 37 either filters the signal inputted therein or increases the DC gain to enhance the performance of the servo system. Third amplifier 38 amplifies the output signal from compensating circuit 37 by such a predetermined amplification amount that the motor can recognize the signal, and delivers the amplified signal as a control signal to motor M.
However, the conventional servo system has the following primary disadvantages:
Because the phase error detector, the rotational speed error detector and the compensating circuit are designed in terms of their hardware, their characteristics are almost always fixed.
In addition, the VCR is made of a similar complex hardware construction in order to correspond to such circuits and various playback conditions such as, for example, a normal playback, a speed change playback and the like. Therefore, due to these many limited features in the servo system design, the system performance is degraded and limited.